This invention relates to a tread brake unit for a railroad car, and in particular, to an improved automatic stroke adjustment mechanism of the tread brake unit.
Known tread brake units of this type in the prior art include the one indicated in Patent Kokai No. 59-192666, and the structure of this tread brake unit will be explained first with reference to the accompanying FIGS. 3 and 4.
There is a brake cylinder 3 on the upper side of the main body 2 of tread brake unit 1. The output of this brake cylinder 3 is transferred from piston 4 to the cylinder main lever 5 extending in body 2 in a vertical direction. The above-mentioned cylinder lever 5 can move forward and backward (a-b direction in FIG. 3) about the fulcrum pin 6 provided at the bottom part of the main body 2. At the same time, the sheath rod 8, which extends in the forward and backward direction, is connected to the spherical bearing 7 at the middle part of the cylinder main lever 5, so that it can rotate around the central axis, and in addition, there is a threaded bore of sheath rod 8. A push rod 9, the front end (left end part) 9a of which protrudes outside the main body 2, is screw-threaded to the bore of this sheath rod 8, and the brake shoe 10 is connected to the front end 9a of sheath rod 8. In addition, the front end 9a is supported by a brake shoe hanger 11 suspended at the side of the upper part of the main body 2 so that it can move forward and backward. In addition, there is a hand brake lever 12 in the upper center part of the main body 2, to move the above-mentioned cylinder main lever 5 manually. Therefore, when the above-mentioned cylinder main lever 5 is moved forward (direction a in FIG. 3) around pin 6, which is the fulcrum, by operating the above-mentioned hand brake lever 12 or by transferring the output of the brake cylinder 3, the sheath rod 8 and the push rod 9 become one and move forward, and because of this, the brake shoe head 10 is pushed against the wheel tread surface, and a specified brake force is generated.
In addition, this tread brake unit 1 is equipped with an automatic adjusting mechanism 20 to maintain the gap between the brake shoe and wheel tread a specified value irrespective of the wear of the wheel and the brake shoe contact surfaces.
This automatic gap adjusting mechanism 20 includes a lever arm 21 which is fixed at the lower end of the above-mentioned cylinder lever 5 and which also extends to the rear of the brake unit: a lever rod 23 which has a roller 22 that rides on the upper surface of lever arm 21, and which can move in the vertical direction, while at the same time moving forward and backward in an interlocked manner with the forward and backward movement of the above-mentioned sheath rod 8; a compression spring 24 which acts downwardly on lever pusher rod 23; a lever 26, the middle part of which is supported by a fulcrum pin 25 so that it can rotate, one end being connected to the above-mentioned lever push rod 23; a pawl 27 which is connected to the other end of lever 26; and a ratchet gear 28 which is press-fit on the circumference of the above-mentioned sheath rod 8. The above-mentioned pawl 27 is pulled in an upward direction by tension spring 29, and because of the spring force of tension spring 29, a side surface of the above-mentioned pawl 27 is pressed onto the tips of the teeth of ratchet gear 28.
With such a structure, when the brake is released, the mechanism assumes the position indicated in FIG. 3 and FIG. 4; in other words, there is a certain gap between the roller 22 at the lower end of the lever push rod 23 and the lever arm 21 of the lower end of the cylinder main lever 5. In addition, the lever push rod 23 is pushed down to the lower moving end by the compression spring 24. When the cylinder main lever 5 moves forward from this state, the sheath rod 8 and the push rod 9 move forward, and the lever arm part 21 of the lower end of cylinder main lever 5 moves upwardly (direction c in the FIG. 3) and touches the roller 22 and pushes the lever push rod 23 in an upward direction against the force of compression spring 24. When the lever push rod 23 moves up in this manner, the lever 26, one end of which is connected to lever push rod 23, rotates around the fulcrum pin 25 clockwise in FIG. 4. Simultaneously, the pawl 27, which is connected to the other end of lever 26, slides down along the tips of the teeth of the ratchet gear 28 against the force of the tension spring 29. In this case, when the shoe gap between the above-mentioned brake shoe 10 and the contact surface of the wheel is within the specified value, the distance moved by the cylinder main lever 5 in the direction a, and the distance moved by the lever arm 21 in the direction c, are less than a certain value. Therefore, when the above-mentioned pawl 27 reaches its lower position, the upper end meshing surface 27a of pawl 27 does not go over the tips of the teeth of the ratchet gear 28 in the specified position. Therefore, in this case, even if the cylinder main lever 5 moves in the direction b and the pawl 27 moves up from its lower position, pawl 27 only slides over the tips of the teeth of the ratchet gear 28, and ratchet gear 28 does not rotate. Also, a spring stop 30 is in contact with the upper part of the ratchet gear 28 with an appropriate pressure, and prevents the unintentional rotation of ratchet gear 28 while the above-mentioned pawl 27 slides over the tips of the teeth of the ratchet gear 28.
On the other hand, when the shoe space becomes more than the specified value as a result of wear on the brake shoe 10 and/or the contact surface of the wheel, the distance moved by the cylinder lever 5 in the direction a, and the distance moved by the lever arm 21 in the direction c, become more than a certain value. Consequently, the above-mentioned pawl 27 reaches its lower position in which the upper end meshing surface 27a of pawl 27 rides over the tips of the teeth of the ratchet gear 28 and into meshing engagement with the teeth. When the cylinder main lever 5 subsequently moves in the direction b, and the pawl 27 is raised from its lower position by the stored spring force, the ratchet gear 28 is rotated counterclockwise in FIG. 4 by the amount of one tooth, and at the same time, the sheath rod 8, in which the ratchet gear 28 is press-fit, also rotates in the same direction. As a result of the rotation of sheath rod 8, push rod 9, which is screw-threaded with sheath rod 8, is extended forward according to the pitch of the screw.
The gap between the brake shoe and the contact surface of the wheel can be kept constant at the specified value by the above-mentioned movement.
A tread brake unit 1 of the above-mentioned type is usually installed on a railroad flatcar 40 having a wheel 41, as shown in FIG. 5. The above-mentioned wheel 41 supports the flatcar 40, as can be seen in the figure via bearing 42 and laminated rubber members 43, therefore giving rise to the problem hereinafter described.
Namely, when tread brake unit 1 operates and the brake shoe 10 is forced into contact with the tread of wheel 41, the laminated rubber members 43 are deformed by this pressure force, and the bearing 42 (axle) moves in the direction of the applied force, so that the stroke of the above-mentioned brake shoe 10 must be greater than before. In addition, with such a support method, the flatcar 40 moves up and down depending on the loading weight (see FIG. 6), and the shoe gap distance L between the wheel 41 and the brake shoe 10, changes. Therefore, to compensate for this too, the stroke of the brake shoe 10 must be increased.
When the stroke of the brake shoe 10 is increased for the reason mentioned above, the distance the cylinder main lever 5 moves in the direction of a in FIG. 3 increases and the distance the lever arm 21 moves in the direction of c in the same figure inevitably increases. In addition, the upper surface of lever arm part 21 is substantially linear, so that the distance the lever push rod 23 moves upwardly increases and the distance the pawl 27 moves downwardly also increases. Therefore, the amount of compression of spring 24 and the amount of extension of tension spring 29 also increases. In this case, both of the above-mentioned springs 24 and 29 act to extend the push rod 9 by rotating the ratchet gear 28 and the sheath rod 8 by the force of the springs when the gap is adjusted. These springs also are subjected to installation space limitations, so that they are both small in diameter and are designed to generate strong spring forces. Therefore, both these springs are usually designed to operate in a range approaching their fatigue limit. Under such operating conditions, when the compression and tension of these spring 24 and 29 increases, as described above, both springs 24 and 29 exceed their fatigue limit strength while still in use, and the springs 24 and 29 are caused to fail prematurely, which is a disadvantage. As can be seen in FIG. 7, when the stress amplitude of spring 24 and spring 29 under operating conditions is plotted on the vertical axis, and the average stress is plotted on the horizontal axis, the fatigue limit curve of said spring becomes the curve indicated by X. Normally, the above-mentioned springs 24 and 29 operate in the position indicated by point A in the figure, but when the displacement of both springs 24 and 29 increases for the reason mentioned above, the fatigue limit of the springs shifts to the point indicated by B, thereby causing premature spring failure, as mentioned above.